Projection printing systems for forming an image on a light sensitive substrate or wafer are particularly suitable for effecting the exposure of photoresist-coated semiconductor wafers in the manufacture of integrated circuits. These systems normally include a photomask or reticle (hereinafter "mask") having a transparent substrate with a pattern of opaque and transparent areas formed on one surface thereof, an illumination system for directing a beam of radiant energy such as light or ultraviolet rays through the mask to the wafer, optical means for forming a focused image of the mask pattern on the wafer and a pellicle for maintaining the images of any dust particles out of focus on the surface of the mask.
A pellicle is a free-standing thin optical film supported on a frame. Normally, the frame is attached to the mask and the optical film stands away from the mask surface by a given distance. Dust particles, which might otherwise settle on the mask and be projected onto the wafer, will settle on the pellicle instead and be defocused. Consequently, when pellicles are used in a conventional projection printing system, one or more dust particles will not affect the yield of a given wafer and yields can be significantly improved.
Ideally, pellicles should be invisible to the radiant energy of the projection printing system. That is, in order to produce clear, well-defined patterns, the optical film of the pellicle should transmit all of the radiant energy used during the projection printing process, with no reflection.
In the past, single layer pellicles, typically made of nitrocellulose, have been used. With increasing miniaturization, the pattern elements have become smaller and the optical transmission of the pellicle films has become increasingly insufficient. In order to increase the optical transmission of pellicle films, that is, reduce their optical reflectiveness or glare, and thus produce clear, well-defined patterns, multilayer pellicles with various anti-reflective coatings have previously been proposed. Such coatings have included fluoropolymers which provided good optical characteristics. However, continuing effort has been directed to the preparation of anti-reflective coatings which exhibit the best possible combination of optical properties and adhesion to the core materials used for pellicles.